SEXUAL slang is going legitimate. In “Carnal Knowledge – Baxter’s Concise Encyclopedia of Modern Sex,” out in February, John Baxter explores the origins of the world’s sauciest terminology. It includes “going commando,” which the author explains is “based on the practice of US Army Special Forces of wearing no underwear on combat missions to avoid ‘crotch rot.’ ” And there’s the long-gone saying once favored by Times Square hookers, “hot lunch,” meaning a “midday sexual encounter” with a john.